DESCRIPTION (Applicant's abstract): The effects of exercise and decreased activity on skeletal muscle have been well characterized. However, much less is known concerning activity-induced adaptations of the neuromuscular junctions that activate skeletal muscle, and the effects of aging on those synapses. This is unfortunate in view of the "graying" of our society, the greater incidence of mobility limiting injuries in the aged, and the increasing participation of our aged population in exercise programs. The overall goal of the proposed project is to shed light on the plasticity of the neuromuscular system to decreased activity, and to an increasingly popular form of recreational and rehabilitative exercise, i.e., resistance training (weight lifting) in both aged and young animals. The specific aims of the research project are: 1)to compare the effects of decreased activity (muscle unloading) on the neuromuscular systems of the young and the aged, 2) to determine the effects of aging on the adaptations of the neuromuscular system to resistance training, and 3) to assess the relationship between the expression of two important synaptic regulatory proteins (NT-4, N-CAM) and the morphological remodeling of neuromuscular junctions in response to resistance exercise, and decreased activity in young and aged muscles. The data gathered will provide important new insight on neuromuscular plasticity to increased and decreased activity in the young and the aged. Perhaps more important, this information will assist in the development of optimal preventative and rehabilitative measures concerning the neuromuscular system in the young and aged alike.